1. Field of the Invention
The present invention relates generally to a screw anchor having a guy-wire-rod threadably secured to a boss within the hub of the anchor. The rod-receiving boss is constructed in a manner that will allow the rod to be disposed at a moderate angle with respect to the axis of the anchor without significantly impairing the ability of the rod or the anchor rod boss to withstand the tension forces imparted by a guy-wire connected thereto.
In a first embodiment, the rod boss is designed to bend longitudinally within certain limits, thus significantly alleviating stresses and strains on the guy-wire-rod and the rod boss when the rod is not axially aligned with the anchor. In an alternate embodiment, the bending moments that would otherwise be imposed on the rod boss and the guy-wire-rod by a nonaligned rod is essentially eliminated through the use of a ball and socket boss construction.
2. Discussion of the Prior Art
It is known to provide a modular screw anchor having an earth-penetrating lead and a one-piece hub and helix which surrounds an upper internally threaded segment of the lead point. An elongated rod of this type of anchor has an end adapted to be threaded into the lead point, and generally has threads at the opposite end for attachment of an eye which facilities connection of a guy wire to the anchor. Exemplary modular anchors are shown and described in U.S. Pat. Nos. 4,334,392 and 4,467,575, both assigned to the assignee hereof.
During installation of the modular anchor as illustrated in the '575 patent, the rod is first threaded into the uppermost end of the lead point whereupon a tubular wrench is then telescoped over the rod in driving engagement with the hub portion of the screw anchor. The wrench has locking dogs on its upper end for engagement with temporary retaining means such as a holding collar or equivalent means threaded on the upper end of the rod to hold the anchor assembly connected to the wrench until the locking dogs are released. Upon connection of the upper end of the wrench to a power source for rotational movement, simultaneous application of downward force and rotational torque on the assembly causes the anchor to be driven into the ground to the desired depth. Similar anchor construction is described and shown in the '392 patent except that the rod may either be integral with the lead point, or threaded throughout the length of the latter. A certain degree of axial misalignment of the rod with the lead point did not have an adverse effect on the integrity of the anchor assembly in most instances because of the f act that the lead point was not integrally attached to the hub and helix.
Screw anchors made up of components which, for example, are welded together presenting a unitary structure made up of a helix, hub and rod-receiving boss however, poses a problem when misalignment of the guy wire with respect to the anchor axis occurs. In this instance, the maximum bending moment which can exist at the threaded joint between the guy-wire-rod and the rod-receiving boss is controlled by the strengths of the anchor rod and boss in the vicinity of the joints.
Furthermore, development of an essentially one-piece cast screw anchor having an integral lead point, tubular hub, peripheral helix, and internally threaded boss cast within the hub for receipt of the threaded end of the guy-wire-rod, also presents the potential problem of undue stresses being placed on the rod/boss joint if the rod is not properly aligned with the axis of the anchor. The one-piece cast anchor is installed in the same fashion as the modular screw anchor. The wrench is telescoped into the hub in surrounding relationship to the rod-receiving boss while the locking dogs at the top end of the wrench hold the anchor assembly in position during installation. Release of the locking dogs allows the wrench to be withdrawn leaving the upper threaded end of the rod exposed for attachment of eye means and a guy wire to the rod.
Typical applications of the rod and earth anchor assembly include stabilization of upright structures, such as for example utility poles, towers for supporting electrical distribution lines, radio and television transmission towers, light standards, and similar structures. When used for these purposes, the anchor is first driven into the ground, and then a respective guy wire is connected to the anchor rod eye. Thus, the relative positioning of the anchor assembly with respect to its guy wire is of critical importance. The anchor assembly and guy wire must be connected and tightened in such a manner that the full benefit of the earth anchor's holding capacity is exploited. This means that the guy wire and the anchor assembly must define, as closely as possible, a straight line from the connection of the guy wire with the tower structure to the tip of the anchor member lead point in the ground. It is widely recognized in the industry that the anchor must be aligned within 10 degrees of the guy wire. In practice, it is standard procedure to specify that anchors be installed with their axis within 5 degrees of the guy wire.
If the guy wire and anchor assembly are not properly aligned as described above, the tightening of the guy wire will adversely affect the anchor rod and the threaded connection of the latter to the rod boss, particularly in instances where the screw anchor is a unitary structure with the rod receiving boss being integral with the anchor hub. This result obtains because as the guy wire is tightened, the end of the anchor rod connected to the guy wire will attempt to align itself with the wire. The rod starts out aligned with the anchor and is pulled out of that alignment in an attempt to align itself with the guy wire as it is tensioned. At the same time, however, the other end of the anchor rod, screwed into the earth anchor hub, will endeavor to maintain alignment with the anchor. This situation is unacceptable, because the rod, in its effort to align itself with the wire, undergoes bending moments.
Furthermore, failure to properly align the anchor rod and guy wire imposes a bending moment and resulting bending stress on the threaded joint between the anchor rod and the rod-receiving anchor hub or boss. The sum of the tensile stresses due to bending and to the applied guy wire load is limited to the tensile strength of the anchor rod. Therefore, the useful guy wire load is reduced by an amount that is directly proportional to the magnitude of the bending stresses, which in turn is directly proportional to the magnitude of the bending moment. It thus follows that reducing the bending moment at the threaded joint will increase useful guy wire load capacity.